Methods for analyzing DNA sequence variation can be divided into two general categories: 1) genotyping for known sequence variants and 2) scanning for unknown variants. There are many methods for genotyping known sequence variants, and single step, homogeneous, closed tube methods that use fluorescent probes are available (Lay M J, et al., Clin. Chem 1997;43:2262-7). In contrast, most scanning techniques for unknown variants require gel electrophoresis or column separation after PCR. These include single-strand conformation polymorphism (Orita O, et al., Proc Natl Acad Sci USA 1989; 86:2766-70), heteroduplex migration (Nataraj A J, et al., Electrophoresis 1999;20:1177-85), denaturing gradient gel electrophoresis (Abrams E S, et al., Genomics 1990;7:463-75), temperature gradient gel electrophoresis (Wartell R M, et al., J Chromatogr A 1998;806:169-85), enzyme or chemical cleavage methods (Taylor G R, et al., Genet Anal 1999;14:181-6), as well as DNA sequencing. Identifying new mutations by sequencing also requires multiple steps after PCR, namely cycle sequencing and gel electrophoresis. Denaturing high-performance liquid chromatography (Xiao W, et al., Hum Mutat 2001;17:439-74) involves injecting the PCR product into a column.
Recently, homogeneous fluorescent methods have been reported for mutation scanning. SYBR® Green I (Molecular Probes, Eugene, Oreg.) is a-double strand-specific DNA dye often used to monitor product formation (Wittwer C T, et al., BioTechniques 1997;22:130-8) and melting temperature (Ririe K M, et al., Anal. Biochem 1997;245:154-60) in real-time PCR. The presence of heterozygous single base changes have been detected in products up to 167 bp by melting curve analysis with SYBR® Green I (Lipsky R H, et al., Clin Chem 2001;47:635-44). However, subsequent to amplification and prior to melting analysis, the PCR product was purified and high concentrations of SYBR® Green I were added. The concentration of SYBR® Green I used for detection in this method inhibits PCR (Wittwer C T, et al., BioTechniques 1997;22:130-1, 134-8); thus, the dye was added after amplification. A dye that could be used to detect the presence of genetic variation including heterozygous single base changes and could be added prior to PCR would be desirable.
Single nucleotide polymorphisms (SNPs) are by far the most common genetic variations observed in man and other species. In these polymorphisms, only a single base varies between individuals. The alteration may cause an amino acid change in a protein, alter rates of transcription, affect mRNA spicing, or have no apparent effect on cellular processes. Sometimes when the change is silent (e.g., when the amino acid it codes for does not change), SNP genotyping may still be valuable if the alteration is linked to (associated with) a unique phenotype caused by another genetic alteration.
There are many methods for genotyping SNPs. Most use PCR or other amplification techniques to amplify the template of interest. Contemporaneous or subsequent analytical techniques may be employed, including gel electrophoresis, mass spectrometry, and fluorescence. Fluorescence techniques that are homogeneous and do not require the addition of reagents after commencement of amplification or physical sampling of the reactions for analysis are attractive. Exemplary homogeneous techniques use oligonucleotide primers to locate the region of interest and fluorescent labels or dyes for signal generation. Illustrative PCR-based methods are completely closed-tubed, using a thermostable enzyme that is stable to DNA denaturation temperature, so that after heating begins, no additions are necessary.
Several closed-tube, homogeneous, fluorescent PCR methods are available to genotype SNPs. These include systems that use FRET oligonucleotide probes with two interacting chromophores (adjacent hybridization probes, TaqMan probes, Molecular Beacons, Scorpions), single oligonucleotide probes with only one fluorophore (G-quenching probes, Crockett, A. O. and C. T. Wittwer, Anal. Biochem. 2001;290:89-97 and SimpleProbes, Idaho Technology), and techniques that use a dsDNA dye instead of covalent, fluorescently-labeled oligonucleotide probes. The dye techniques are attractive because labeled oligonucleotide probes are not required, allowing for reduced design time and cost of the assays.
Two techniques for SNP typing using dsDNA dyes have been published. Allele-specific amplification in the presence of dsDNA dyes can be used to genotype with real-time PCR (Germer S, et al., Genome Research 2000;10:258-266). In the method of the Germer reference, two allele-specific primers differ at their 3′-base and differentially amplify one or the other allele in the presence of a common reverse primer. While no fluorescently-labeled oligonucleotides are needed, genotyping requires three primers and two wells for each SNP genotype. In addition, a real-time PCR instrument that monitors fluorescence each cycle is necessary.
The other dye-based method does not require real-time monitoring, needs only one well per SNP genotype, and uses melting analysis (Germer, S, et. al., Genome Research 1999;9:72-79). In this method, allele-specific amplification is also used, requiring three primers, as with the previous Germer method. In addition, one of the primers includes a GC-clamp tail to raise the melting temperature of one amplicon, allowing differentiation by melting temperature in one well. Fluorescence is monitored after PCR amplification, and real-time acquisition is not required.